Glass bubbles having an average diameter of less than about 500 micrometers, also commonly known as “glass microbubbles”, “hollow glass microspheres”, “hollow glass beads”, or “glass balloons” are widely used in industry, for example, as additives to polymeric compositions. In many industries, glass bubbles are useful, for example, for lowering weight and improving processing, dimensional stability, and flow properties of a polymeric composition. Generally, it is desirable that the glass bubbles be strong enough to avoid being crushed or broken during processing of the particular polymeric compound. Glass bubbles have been incorporated into rubber compositions. For example, glass bubbles have been incorporated into rubber compositions for shoe outsoles to lower the weight of the rubber composition. See, e.g., Korean Patent Nos. 100894516, published Apr. 22, 2009, and 100868885, published Nov. 17, 2009.